Laser fuses can be used to rewire memory and logic circuits. For example, in dynamic or static memory chips, defective memory cells may be replaced by blowing fuses associated with the defective cells, and activating a spare row or column of cells. This circuit rewiring using fusible links allows considerable enhanced yields and reduces the production costs. Also, logic circuits may also be repaired or reconfigured by blowing fuses. For example, it is common to initially fabricate a generic logic chip having a large number of interconnected logic gates. Then, in a final processing step, the chip is customized to perform a desired logic function by disconnecting the unnecessary logic elements by blowing the fuses that connect them to the desired circuitry. Still other applications of laser-blown fuses are possible.
Although deep sub-micron technology is advancing such that transistors and interconnects are continuing to shrink in size, the size of laser fuses is becoming limited by the resolution of the laser machine. Historically, the laser fuse design rules have shrunk less compared to those of transistors and interconnects. Hence, the chip area occupied by laser fuse continues to increase as deep sub-micron technology advances.
Conventional fuse structures include a fuse window, a laser fuse disposed below the fuse window, and a protective guard ring formed around the fuse. The fuse area is typically defined by the area within the guard ring. Unfortunately in such conventional fuse designs, no circuits can be placed directly beneath the fuse area because the guard ring and laser fuse occupy this chip area.
Accordingly, a fuse structure is needed that saves chip area under the laser fuse area which can be used for routing circuits.